Miss Nicholson, who revealed that she had rejected overtures from Labour to join it in the summer, criticised Lord Tebbit, the former Conservative party chairman, Michael Heseltine, the Deputy Prime Minister, and the Government whips.
However, Downing Street and Conservative Central Office insist that the Government will not be driven into an early general election. Although the Tory majority is likely to fall to one, the Ulster Unionists will probably abstain or back the Government in any future vote of confidence, allowing it to scrape through. The Ulster Unionists' deputy leader, John Taylor, said that his party would not force a premature election, providing the Government "continues governing in the best interests of the UK in general and Northern Ireland in particular". Ken Maginnis, their Defence and Home Affairs spokesman, said that the defection had changed nothing.
The biggest worry for the Tories is that there will be more defections. Miss Nicholson said she had held many private conversations with backbench colleagues, and "their replies to me remain with me".
No Tory backbencher would speak openly yesterday, but one said privately that Labour now represented the country better than his own party. Only the thought that "I couldn't kick my constituency workers in the teeth" kept him from resigning the whip.
Miss Nicholson said yesterday: "I spent a long time trying to discover a different Conservative Party, that the one-nation Conservative Party I cared for very much indeed still existed and was still in business; that it did still care about people from different backgrounds, different ethnic minorities, but it isn't so.
"I should have known it was not so. I got heavily criticised by Norman Tebbit, when I was vice- chairman of the Conservative Party, for daring to go out and recruit Asian and black women. I got the most incredible barracking for that."
She was clearly angered by the implication from Mr Heseltine that she was motivated by pique at missing out on promotion. He said she had asked twice in recent months for preferment.
She said: "If the Conservative Party can't use people like me in some capacity, even stuffing envelopes, then they really have changed. I felt they had. For a long time I blamed myself. I felt that I had gone soft, woolly or weak. But it is the Conservative Party that has changed."
Mr Heseltine, she said, "knows something about resigning on principle. He ought to share a commonality of vision on this". When pressed futher on her contacts with the Deputy Prime Minister, she insisted the Conservative party was very hierarchical: "People of his magnitude were not for the likes of me."
She also criticised the government whips. She said when she voted against John Major's preferred option on the Nolan report, she was "exhaustively rubbished by the whips' office".
Miss Nicholson added: "I explained that it mattered a lot to me that Parliament should be cleaner than clean, particularly with the abysmal public attitude towards Parliament. I voted that way and got pulled apart. It means the freedom to vote is an absolute nonsense."
Lord Tebbit described Miss Nicholson's account as a "remarkable figment of imagination". He said: "The problems she had with me sprang from her desire to spend money on her office and incur financial costs unrelated to the quality of the work she was performing. It was part of my job at Conservative Central Office to build links with groups representing Asian minorities. In that I was successful."
Mr Heseltine tried to damp fears about further defections - Miss Nichsolson's "unsubstantiated claims" were not enough, he said - but government sources conceded there were worries over another "one or two" disgruntled Conservative MPs.
Leading figures on the centre-left of the party, who once supported Mr Heseltine, said they were profoundly disillusioned about the "lurch to the right".
One senior left-winger said he was tired of Mr Major appealing to "base prejudice, and the idiot, insolent and arrogant right" and warned that the loyalty of the party's left could soon snap.
Paddy Ashdown, leader of the Liberal Democrats, said the sooner an election was called, the better. His campaign co-ordinator, Lord Holme, predicted the Conservatives would be unable to survive the coming year.
Labour sources said an overture to Miss Nicholson from their benches was "news to us" and that Tony Blair was not involved.
Meanwhile Mr Major said in an interview recorded before the defection: "I have no intention of running precipitately for a general election."
He told Radio 4's Today programme: "I would not expect a general election before 1997."
Asked if he would soldier on if he lost his overall majority, Mr Major replied: "Of course. I was expected to do a particular job and I expect to complete that job."
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